Before we look at your build, here are some things to think about. If you're just wanting to make something to see how much damage you can do, disregard most of this. I'm assuming you want to win.
1. Previous posters were correct, you absolutely have to win initiative.
2. They were also correct about action economy. You absolutely have to overcome it.
3. You know about their builds. Do they know yours? If I knew I was facing a magus, I'd have a ring of evasion, resist energy, and probably spell resistance too.
4. Do you know where and under what circumstances you'll be facing them? Day, night, city, forest, etc? Will you have time to buff? Is there a time limit or space limit on the battle itself?
5. Do they frequent these forums? If they do, and they didn't know about your build before...well they do now. Also they'll know any advice you receive.
6. One good thing is you can build 100% for this one fight. Nothing else matters. And you don't have to worry about conserving resources either.
Now let's look at your opponents.
1. What are their strengths? All three are likely 90-100% ranged attackers (not counting the animal companion) and all probably do massive damage. And of course that means they can full attack from wherever. How can you avoid that?
2. What are their weaknesses? Will, Fort, or Reflex saves? AC or CMD? What's the best way to target them?
3. Do they get to rebuild like you do? If so, what is their level of system mastery and strategic ability?
Let's talk about strategy.
1. Don't do what they expect. This is especially true if they know what you're playing.
2. Identify your greatest threat. This isn't necessarily the one who can do the most damage. If one of them can cripple you, or rescue his teammates, that might be the greater threat.
3. Make every action count. Be effective without being wasteful. You don't need to do 300 damage every attack. Any damage over the amount needed to kill them is wasted. Although if you go in for the kill, it needs to succeed.
4. You can't afford to trade attacks. Even if you kill three of them (counting the animal companion), you might still lose the fight. You need hit and run tactics. Also, not every attack has to kill. If you can irreparably incapacitate them, that's an effective attack.
5. Pay attention to the little things too. AoOs and readied actions are their friends not yours. What items might completely hose you if they had it and how can you adjust for that?
6. Assume that they've read all of this and have adjusted accordingly. Assume their system mastery is equal or greater than yours.
7. Manage your resources wisely. Money isn't your only resource. Actions are a resource. Feats are a resource. Health is a resource. Speaking of which, if allowed, blood money is your friend.
One potential use, if you can get access to them, is all the symbol spells. ALL OF THEM. Just wear a covered wood plank with all the spells on it. Move action remove cover, watch your enemies make 18 different saves. Well, ideally watch them fail 18 different saves. There are actually a few spells with material components that might be worth the cost even without blood money. Visualization of the Mind/Body are pretty boss
Finally, your build. There are 6 categories to look at: sensory, initiative, action economy, movement, defense, and offense. But first,
1. Effectiveness and synergy are key. What feats, items, classes give you the most bang for your buck? You know your INT is going to be high, probably DEX too. How can you leverage those for more?
2. Why sensory? Can't do anything if you can't see anything. Can't attack, can't defend. Fortunately this is one the easiest to deal with. Max perception (plus whatever resources you can squeeze into it), get see invis or true seeing and maybe scent, tremorsense, or blindsight/sense because stealth is a thing.
One potentially amazing trick here is a Goz Mask. Useful if your opponents use mist/fog spells but even better if you use them. Few people carry with them a means to see through fog by default (if you do this, be sure to choose a spell or item that they can't easily counter; bonus points if you have necklace of adaptation or similar and use debuff clouds).
3. Already touched on initiative but it's probably worth repeating. Go first or die. Kensai, as someone mentioned is a really good choice.
4. Also already talked about action economy but it's definitely worth expanding on. You'll want to spend resources boosting your action economy. If possible, take leadership. Additionally, if possible, trade the black blade for a familiar (improved). You really, really need more actions. I'm going to assume neither of those is possible. However there are various things (items especially) that you can use for a bit of a boost.
Some things give you a direct boost, e.g. quick runner's shirt, staggerproof boots, corset of delicate moves, quickened rod. A few class abilities, feats, and spells out there can help too. Too many to list really. Just pick wisely.
Then there are a few spells, feats, abilities, items (like Companion Figurine or summons) that get you some added actions. One interesting trick you could try is to purchase some dire lions (or whatever scary beastie you can buy or tame). Cast Carry Companion on all of them, put them in a glass orb. Drop the orb at the start of combat as a free action and watch your enemies freak out as a dozen fierce critters appear in front of them. Distraction, meat shields, battlefield control, damage? Check. Action? None. Just maybe, ya know, make sure you taught them not to attack you first.
Related to this, shrink item (or a security belt) can really really ruin someone's day. Make a giant double walled crate. In between the walls, put nails, ball bearings, whatever. Inside the main space? Gunpowder. Lots and lots of gunpowder (the gunsmithing feat lets you make it at 1/10 cost). With a security belt, move action remove item, free action drop, swift action GTFO, standard action boom (or reverse the swift/standard actions). You know how much gunpowder is in 10 cubic feet? It's something like 350d6 worth of fire damage (for 7000g). Low reflex save but who cares? With all that shrapnel a nice GM would up the save and radius and make it half fire, half OMG-WHAT-DID-YOU-DO?!
5. Of course you'll want a way to fly and teleport/dim door and what not but consider some alternate methods too. Etheral/incorporeal travel and earthglide can really wreck someone's day if they aren't ready for it (and there's a couple things that give it). Say you pop some fog up, hit 'em from the mists, and then they get rid of the fog only to find you've vanished. They're going to assume you're invis or teleported but then BLAM, full attack from right beneath them. You'd need tremorsense and blind-fight/improved blind-fight (conveniently found in item and spell form) to target them but they won't be able to target you back.
6. There's a reason I put these in this order and it is because most of your defense should come from good tactics. Don't let them find you and DEFINITELY don't let them catch you! This is where those movement options come in. But also fog spells, invisibility, stealth, trickery. I'm a big fan of Hide in Plain Sight and it's easier to get cousin Hellcat Stealth...just flood the area with light and watch them squint and lose track of you. Just make sure your GM follows reasonable rules for HiPS and stealthing if you go this route.
However, there's always those pesky readied actions or unlucky rolls that leave you exposed. So let's go down the list.
AC is a sucker's game, especially with a damn gunslinger as an enemy. You'll get a lot farther with mirror image and displacement. Blink would be a good option as well but it can ruin your day too. Depends on what you use, really.
DR and SR are definitely worthwhile and there are a few ways to get 'em. Resists are a little less necessary although if the sorc has a tendency toward one particular element it wouldn't hurt. Some kind of emergency healing would also be a good idea.
Obviously get good saves (although I wouldn't spend a feat on it). Consider a ring of evasion but, again, tactical positioning should prevent this. Definitely invest in a few of the items/traits that let you reroll saves.
One thing to consider here: silent image/major image/whatever. If your enemies start readying actions, a common trigger is "when x appears/attacks". A wand of silent image (ideally used by a summon or familiar) can make them waste their resources while conserving yours. Also doubles as extra defense. Flood the area with images and play Where's Waldo? except Waldo in this case is waiting to stab them when they aren't looking
7. Now the fun stuff. Offense! Some of the others have already mentioned some good damage dealing builds (and again, if your opponent's know you're a magus that could be a trap) but I'd actually recommend something else.
Usually, in cases like this, I'd say avoid the save or die stuff but in this case the Hexcrafter Magus with the slumber hex is a strong contender. Quickened Ill Omen (or someone with a wand of it) + Slumber will 9.9/10 times take someone out of the fight for at least a round. Normally that wouldn't be a super effective way of dealing with superior numbers (after all, they can still be woken up). However, IF you can squeeze a full round in for either you or a buddy, a coup de grace is practically certain death. There's an item that prevents AoO while CdGing but really what you want is to separate your enemy. Divide and conquer. Stone walls maybe or just taunt/trick them into splitting up. Alternately a good fog cover + Goz Mask. They can't wake/protect their friend if they can't find them.
Also, putting slumber on someone flying high enough can be pretty spectacular.
Alternately, look at your options and consider: what would my opponents LEAST expect me to do that still has a good chance of killing them?
